Field
The embodiments disclosed herein relate to a system and method for controlling emission of Volatile Organic Compounds and, more specifically, to an improved system and method for increasing volumetric throughput through an internal combustion engine used to control Volatile Organic Compounds.
Description of the Related Art
The direct release of Volatile Organic Compounds (VOC's) into the atmosphere has been for some time now recognized as a primary contributing factor in affecting ozone concentrations in the lower atmosphere. The EPA has established standards for safe levels of ozone, and local air quality districts have implemented regulations and mandated control measures pertaining to the release of hydrocarbon vapors into the atmosphere, from operations such as soil remediation, storage tank inerting, and storage vessel loading and unloading; that have been identified as sources of hydrocarbon emissions responsible for impacting ozone levels.
The process of treating these vapors, through any of a variety of methods, is typically referred to as “degassing”; which is either the collection or on-site destruction of these vapors as an environmentally responsible alternative to their otherwise direct release into the atmosphere.
The internal combustion engine, as well as open-flare incinerator units, have been employed for several decades as a means of on-site destruction of these VOC's by elemental combustion. The combustion process does itself give rise to the undesirable production of carbon monoxide and nitrogen oxides; however this has been accepted as a reasonable consequence for the nearly 99% efficiency in the destruction of hydrocarbon based VOCs. These consequential emissions are accepted, but tolerated only to a regulated extent, and are also a factor to be considered in engines and incinerators employed in vapor destruction applications. The maximum permissible limits of consequential hydrocarbon, carbon monoxide and nitrogen oxide emissions are regulated to different standards within different air quality regions.
The many different VOC's typically subject to treatment represent a wide range of hydrocarbons between C1 through C10 along with their corresponding alcohols and ketones. Each of these individual compounds is characterized by having unique upper and lower flammability limits, expressed as a range of concentration in atmospheric air within which a source of ignition results in combustion of the mixture. This data is well established and widely published; along with the stoichiometric mixture ratio for each of these compounds, defined as the theoretically ideal mixture at which combustion will be complete without a remaining excess of either air or fuel. Generally, combustion is most complete with a slight excess of air, approximately 15%, being slightly leaner than the theoretical stoichiometric mixture concentration.
The internal combustion engine, as well as open-flare incinerator units, have been employed for several decades as a means of on-site destruction of these Volatile Organic Compounds by elemental combustion; but each with slightly different performance characteristics. In the case of the open-flare incinerator unit; the VOCs to be processed are typically introduced at a vapor concentration equivalent to or less than the lower explosion limit and passed over a continuously maintained flame source responsible for combustion of the subject VOCs. Open-flare incinerator units are able to support combustion of the introduced VOCs at concentrations less than the lower explosion limit by virtue that the flame front has already been established by a continuously maintained pilot flame. The disadvantage of the open-flare incinerator unit is that the inlet concentration is limited by the ability of the unit to dissipate the amount of heat generated by the combustion of VOCs, based upon the heat content of the VOCs undergoing treatment. A further disadvantage of open-flare incinerator type units is that greenhouse gasses (CO2) are continuously generated by the pilot flame without relation to the mass quantity of VOCs being processed.
In the case of the internal combustion engine employed in vapor destruction applications, the resultant heat produced from combustion of the VOCs is very effectively handled by the engine cooling system, affording VOC concentrations in the upper range approaching the upper explosion limit. The internal combustion engine is also self-sustaining in that the fuel source is entirely that of the subject VOC itself and does not require any addition of fossil fuel until the concentration of subject VOC falls below the lower explosion limit of the subject VOC undergoing treatment.
It is important to note that the aforementioned values of lower and upper explosion limits as defined for subject VOC's undergoing treatment was established in a laboratory setting and under the thermodynamic principles of a constant pressure (Cp) type process. Actual conditions of combustion within the internal combustion engine, whether spark ignited or compression ignited, more closely resemble combustion characteristic of a “constant volume” (Cv) type process and typically must therefore be adjusted. This adjustment is on the order of approximately 15% above the lower explosion limit (herein defined as the lean limit), and approximately 15% below the upper explosion limit (herein defined as the rich limit).
Because the many VOCs typically subject to treatment are too numerous to elaborate herein, gasoline vapor has been selected as the model for purposes of discussion. In selecting gasoline vapor; it is herein defined as having an upper explosion limit of 7.5%/vol ; with a lower explosion limit of 1.5%/vol ; and a stoichiometric value of 4.5%/vol. Adjusting these values for practical combustion ranges within the internal combustion engine operating under the thermodynamic principals of a constant volume combustion process ; the rich limit is defined as 6.5%, the lean limit as 2%, and the stoichiometric ratio as 4.5%. There are many factors which could influence these specific values; but these values are selected for the purpose of discussion herein.
The internal combustion engine employed in vapor destruction applications has traditionally been that of the “lean-burn” type ; wherein the process vapor is introduced at a concentration value less than stoichiometric and more closely approximating that of the lean-limit for the subject VOC. At this lean mixture, the resultant emissions with regard to hydrocarbons and carbon monoxide, tend to be at their lowest value, and remain low up to the lean-limit where after combustion is no longer possible. Oxides of nitrogen emissions tend to increase dramatically on the immediate lean side of stoichiometric, but then fall in value as the mixture becomes increasingly lean up to the lean limit. These engines are typically fitted with “reduction/oxidation” type catalytic convertors as a final polish to the exhaust stream prior to emitting into the atmosphere.
Although lean-burn operation is a sought after objective for modern engines employed in power producing applications, such as motor vehicle and industrial power applications wherein fuel efficiency and minimal exhaust emissions are of primary concern; this is not the ideal configuration for such engines employed in vapor destruction applications wherein maximum fuel consumption in terms of vapor processing volumetric throughput are of primary interest.